Dibenzothiophene oxide or dioxide sensitizers for organic photoconductors

ABSTRACT

Electrophotographic elements utilizing a layer containing an organic photoconductor, e.g. poly-N-vinylcarbazole, are provided with enhanced light-sensitivity together with the other properties required for electrostatic photocopying by incorporating in said layer as an electron acceptor a dibenzothiophene oxide, or dioxide, that forms an electron donoracceptor complex with the photoconductor. A preferred acceptor is 1,3,7-trinitro-dibenzothiophene-5,5-dioxide, a new compound.

United States Patent 11 1 Crommentuyn et al.

154] DIBENZOTHIOPHENE OXIDE OR DIOXIDE SENSITIZERS FOR ORGANIC PHOTOCONDUCTORS [75] Inventors: Gerardus J. Crommentuyn, Lottum;

Joannes A. Hagendoorn; Hendrik Knibbe, both of Venlo, all of Netherlands [73] Assignee: Oce-van der Grinten N.V., Venlo,

Netherlands [22] Filed: Sept. 5, 1973 21 Appl. No.: 394,458

[30] Foreign Application Priority Data Sept. 15, 1972 Netherlands 7212511 [52] US. Cl 96/15; 96/16; 260/329 F [51] Int. Cl. G03G 5/06 [58] Field of Search 96/15, 1.6

[56] References Cited UNITED STATES PATENTS 3,174,854 3/1965 Stumpf et a1. 96/1.5

[4 1 Sept. 16, 1975 3,287,114 11/1966 Hoegl 96/1.5 3,421,891 1/1969 lnami et a1... 96/l.6 3,545,969 12/1970 Herrick et al.... 96/].5 3,547,627 12/1970 Amidon et a1. 96/1.5 3,615,384 10/1971 Gipstein et a1 96/15 X 3,617,271 ll/1971 Poot et a1. 96/15 3,655,378 4/1972 Contois et a1 96/1 .5

Primary ExaminerRoland E. Martin, Jr. Attorney, Agent, or FirmAlbert C. Johnston 57 ABSTRACT 15 Claims, N0 Drawings DIBENZOTHIOPHENE OXIDE OR DIOXIDE SENSITIZERS FOR ORGANIC PHOTOCONDUCTORS The invention relates to an electrophotographic reproduction element, comprising a suitable support with a photoconductive coating layer on it, which contains a donor-acceptor complex, built up of an electron donor having photoor semiconductor properties and an electron acceptor, whereby the acceptor itself also may or may not possess photoconductive properties.

Further the invention relates to a process for preparing a photoconductive composition which is suitable for the preparation of an electrophotographic reproduction element according to the above-mentioned composition, whereby a mixture is prepared of an organic electron-donor compound possessing photoor semiconductor properties and an organic electronacceptor compound, by which a donor-acceptor complex is formed and whereby the acceptor itself also may or may not possess photoconductive properties.

Photoconductive compositions which contain an organic compound as photoconductive element, either monomer or polymer, are generally known. Examples of monomeric compounds which have been proposed as photo-conductors are: anthracene, phenanthrene, pyrene, perylene, carbazole, substituted carbazole, oxadiazole, substituted oxadiazole and other aromatic, mainly polynuclear compounds with or without one or more hetero atoms in an aromatic ring. Examples of polymeric compounds which have been proposed as photoconductor in literature are: poly-1 and poly-9- vinylanthracene, polyacenaphthylene, poly-N- vinylcarbazole, as well as their substituted produces such as nitrated or halogenated poly-N-vinylcarbazole, especially poly-3-bromo and poly-3,6-dibromo-N- vinylcarbazole, as well as copolymers, for instance of 3-nitro-N-vinylcarbazole and acenaphthylene.

However, all organic photoconductors known so far possess a light-sensitivity which is by far inferior to that of the usual inorganic photoconductors; Se and ZnO.

Consequently, until recently no organic photoconductor had found commercial application. In the course of years continuous efforts have been made to improve the light-sensitivity in such a way that practical application became possible, by means of sensitivityincreasing compounds, such as sensitizers, by the addition of which. the absorption spectrum is moved towards the visible part of the electromagnetic radiation, and activators by which the light-sensitivity at a certain wavelength; is raised. Although for a long time these efforts remained without success, it has appeared of late that some important progress has been made, Especially by selecting as sensitivity-increasing compound an electron accepting compound which can form a donor-acceptor complex (pi-complex or charge transfer complex) with an electron-donating photoconductive compound, a photoconductive layer can be obtained which is so light-sensitive that electrophotographic reproduction elements can be made with it. Contrary to what can be concluded from older patent specifications, for instance from the German Auslegeschriften 1,111,935, 1,127,218 and 1,219,795, it appears that the proportion donor: acceptor must be about 1 l, as follows from the Dutch patent application No. 7013324, laid open to public inspection, from the German patent application No. 1,572,347, as well as from the article of R. M. Schaffert in I.B.M. Journ. Res. Develop. 15, (January 1971). Well known complex forming acceptors are for instance tetracyanoethylene (1.38), chloranil (1.38), 2,3-dichloro-5,6- dicyano-p-benzoquinone (1.47), p-benzoquinone (0.75) and 1,3,5-trinitro-2-chlorobenzene picrylchloride) (0:58). The number between brackets behind the compound represents the measured electronaffinity (EA) expressed in eV.

However, the best known and mostly applied com plex forming acceptor is 2,4,7-trinitrofluorenone, which especially with the structurally allied carbazole or poly-N-vinylcarbazole yields a very light-sensitive combination. Recently also electron accepting compounds derived from fluorenone were described as sensitivity-increasing compounds. This relates to the substituted or non-substituted 9- dicyanomethylenefluorene compounds, for instance from the British patent specifications 1,245,008 and 1,222,252. Although this does not appear from the two British patent specifications, the above-mentioned 9- dicyanomethylenefluorene compounds are photoconductive by themselves, as follows from the articles of Tapan K. Mukherjee in J. Phys. Chem. 70, nr. 12, 3848-52 (1966) and 73, nr. 12, 4381-2 (1969).

The mechanism by which this sensitivity increase is formed, i,e, whether the complex formed is photoconductive by itself or has a sensitizing and/or activating effect, is not known. Therefore, the sensitivity increasing compound, which as a result of its electron accepting properties can form a pi-complex with photocon ductive electron donors, will henceforth be indicated neutrally as an acceptor.

For all that there are, practically speaking, only two classes of pi-complex forming compounds which are considered as acceptor for poly-N-vinylcarbazole --which up to now has been the most attractive and most applied photoconductornamely the above mentioned fluoreneand fluorenone compounds. The compounds which are most considered and also suitable for practical application out of the two classes are the 2,4,7-trinitro derivatives.

It will be clear without more that which such a limited assortment it is difficult to develop a reproduction element, based on organic photoconductors, which in all facets will function in an optimum way.

The invention makes it possible to enlarge the assortment of acceptors for organic photoconductive donors, especially poly-N-vinylcarbazole.

The object of the invention is to provide an electro' photographic reproduction element which in practice gives as much satisfaction as the known elements.

This is achieved by selecting an electrophotographic reproduction element, which comprises a suitable sup port with a photoconductive coating layer on it, which layer contains a donor-acceptor complex built up of an electron-donor having photo or semiconductor properties and an electron-acceptor, whereby the acceptor itself may or may not possess photoconductive properties, characterized in that the acceptor is a compound of the formula in which n 1 or 2 and the symbols R and R, which can be the same or different, each represent one or more hydrogen atoms or one or more electronattracting groups, known by themselves. The compounds in which n 1 are indicated as dibenzothiofene-5- (mono)-oxides, those in which n 2 as dibenzothiofene-5,5-dioxides.

Although some of the dibenzothiofene oxides and dioxides now proposed were already known compounds by themselves, especially in pharmacy, as appears from Pharm. Bull. Tokyo, part 5 (1957) page 548-52, the person skilled in the electrophotographic process was not conversant with their property to accept easily electrons of other compounds, by which they can form donor-acceptor complexes with these compounds. Their application as complex-forming acceptor in the electrophotographic process of course was not known either and, as can be concluded from the following, was also not obvious. For, in order that a donor-acceptor complex can be formed, the one compound indeed will have to give away electrons (donor) and the other will have to pick up electrons (acceptor). 1f the donor is kept the same, the measure of complex-formation will only depend on the acceptor. ln proportion as the electron-affinity (EA) of the acceptor is higher, a quicker and/or stronger complex-formation will occur. lt is supposed that besides the EA factor also other, especially steric, factors play a part in the complex formation, for instance whether donor and acceptor are well fitting to each other, so that they can approach each other so close that their mutual tendency to give away electrons, respectively to pick up electrons, also indeed leads to this result.

2,4,7trinitro-9-fluorenone (TNF) is a good, active acceptor. Its EA is 1,18 eV. When looking for new acceptors, the object should be to find an acceptor which gives as much satisfaction as TNF; acceptors of inferior properties are not needed. On the basis of the fact that 2,4,7-trinitro-9-dicyanomethylenefluorene (DTF) has the same spatial structure as TNF and has an EA of 1.44 eV, it may be expected that DTF is a better acceptor than TNF.

Practice proves that this expectation is right: photoconductive layers consisting of PVK DTF have a considerably higher light-sensitivity than layers built up of PVK TNF.

Based on the fact that l.3,7-trinitro-dibenzothiofene- 5,5-dioxide, the formula of which is has the same spatial structure as TNF and has an EA of only 0:98 eV, it might be expected that this dibenzothiofene-dioxide (DBTO is a worse acceptor than TNF. This appears not to be the case in practice: photoconductive layers built up of PVK and DBTO whereby the proportion donor acceptor is about 2 1, are equally light-sensitive as photoconductive layers of PVK and TNF in the same proportion.

The invention adds a new class of acceptors, very suitable for the electrophotographic process, to the two already existing classes. Especially in combination with poly-N-vinylcarbazole the compounds of this new class of acceptors yield an electrophotographic reproduction element which, with regard to all properties important for a reproduction element, such as discharge speed in light and dark, memory effect and contrast reproduction, can excellently stand a comparison with the best reproduction elements known so far based on organic photoconductors.

The following enumeration of dibenzothiofene oxides and dioxides may serve as illustration of the acceptors suitable according to the invention:

3-nitro-dibenz othiofene-5-oxide 3,7-dinitro-dibenzothiofene-5-oxide 3,7-dinitro2,8-dibromo-dibenzothiofene-S-oxide 2-nitro-dibenzothiofene-S,S-dioxide 3wnitro-dibenzothiofene-S,5-dioxide 2-bromo-dibenzothiofene-S,5-dioxide 3-bromo-dibenzothiofene-S,5-dioxide 4-bromo-dibenzothiofene-S,5-dioxide 3,7-dinitro-dibenzothiofene-5,5-dioxide 2,8-dinitro-dibenzothiofene-5,5-dioxide 2-bromo-7-nitro-dibenzothiofene-S,S-dioxide 3-bromo7-nitro-dibenzothiofene-5,5-dioxide 2,8-dibromo-dibenzothiofene-5,5-dioxide 3,7-diobromo-dibenzothiofene-S,S-dioxide 3,7-dicyano-dibenzothiofene-5,5-dioxide l,3,7-trinitro-dibenzothiofene-5,5-dioxide 3,7-dibromo-1,9-dinitro-dibenzothiofene-5,5-dioxide 2,3,7,8-tetrachloro-dibenzothiofene-5,5-dioxide It is supposed that the complex formation and therefore the sensitivity increasing effect of the dibenzothiofene oxides and dioxides is dependent on the electronattracting effect of the sutstituents: the complex formation and the sensitivity increasing effect augment in proportion as the electron-attraction becomes stronger. Therefore the strongly electron-attracting nitro groups are preferred as substituents.

On the average the effect of the dioxides is greater than that of the mono-oxides, for which reason the dioxides are preferred. Among the dioxides themselves the new 1,3,7-trinitrodibenzothiofene-S,S-dioxide, which has not yet been described in literature, takes the first place, so that preferably this compound is used as acceptor.

In principle the above mentioned acceptors can be applied with each electron-releasing photoconductor with which a donor-acceptor complex can be formed.

Especially with poly-N-vinylcarbazole, substituted if necessary, a very effective complex is formed, but also with monomeric carbazole compounds and other photoconductors, such as 2,5-bis (4-diaminphenyl)- and 2,5-bis (4-dialkylaminophenyl)-1,3,4-oxadiazole or poly-9-vinylanthracene, substituted or not substituted, light-sensitive donor-acceptor complexes can be formed. 7

The quantity of acceptor added, related to the quantity of photoconductor, can vary within very wide limits. The lower limit is determined by that quantity which still exercises an activating effect on the photoconductor. This lower limit appears to lie near the limit of 0,001 mole per 1 mole of photoconductor, as already mentioned in older literature, calculated according to the monomeric unit.

The upper limit is determined by the solubility of the acceptor in the solvent. This upper limit lies for instance near the mole proportion 1 1 for the combination preferably applied, PVK/1,3,7-trinitrodibenzothiofene-5,5-dioxide.

The quantities of acceptor preferably applied lie in the range between :02 and 1 mole per mole of photoconductor, calculated according to the monomeric unit. The reproduction elements according to the invention are obtained by coating a suitable support with a photoconductive composition containing a donoracceptor complex according to the invention. The support can for instance consist of a metal plate or foil, a plastic foil on which a conductive layer is applied or of paper which is either sufficiently conductive by origin, for instance by impregnation in the paper mill with conductive materials, or has been made conductive by providing it with a conductive layer, for instance a layer containing carbon.

Besides the electron donor and the electron acceptor according to the invention, the light-sensitive layer may also contain one or more photoconductors and/or one or more light-sensitivity increasing compounds.

If the electron donor or acceptor itself does not possess film forming properties, a binding agent can be added for that purpose.

It has appeared by experiments that the lightsensitivity of photoconductive layers according to the invention is dependent on the polarity of the charge and on the proportion donor-acceptor in the complex. Thus photoconductive layers, consisting of PVK/1,3,7- trinitro-dibenzothiofene-5,5-dioxide in the mole proportion of about 1 1 to about 1, if negatively charged, are clearly more sensitive than when they have been charged positively. At proportions of about 10 l to 25 1 the layers are in both cases almost equally sensitive, whereas layers in which the proportion donor acceptor is greater than about 25 1, when positively charged, are more sensitive than when they have been charged negatively. The thickness of the photoconductive layers is not critical and can vary within fairly wide limits. In general a thickness of 3-10 ,u. is already sufficient to yield layers which are suitable for practical use, but also thicker layers, for instance of ,u, are suitable.

The reproduction elements according to the invention can be used both in the direct and in the indirect electrophotographic process. In the direct electrophotographic process the latent image formed on the reproduction element is developed and fixed. In the indirect electrophotographic process the latent image is first transferred onto a receiving material, such as pa per, and is developed and fixed on it, or is first devel oped after which the loose powder image is transferred to a receiving material and is fixed on it.

The invention further relates to a process for the preparation of a photoconductive composition suitable for the manufacture of a reproduction element as herein set forth, whereby a mixture is prepared of an organic electron donor compound which has photoof semiconductor properties and an organic electron acceptor compound, by which a donor-acceptor complex is formed, whereby the acceptor itself may or may not possess photoconductive properties. This process is characterized by the fact that as acceptor a compound of the above formula Inn is taken, in which n 1 or 2 and the symbols R and R, which may be the same or different, each represent one or more hydrogen atoms or one or more electron attracting groups known by themselves.

The preparation of these new acceptors isrealized with the aid of reactions which are usual in organic chemistry. Thus, starting from dibenzothiofene, the mono-oxides can be prepared by oxidation with chlorine in carbon tetrachloride, as described in J. Am. Chem. Soc. 70, 1748 (1948), after which by nitration the monoand dinitrocompounds are produced. The dioxides can be synthetized by allowing dibenzothiofene to oxidize with a mixture of sulfuric acid and dichromate. By means of nitration the mono, dior trinitro compound can be obtained. When the dibenzothiofene from which one starts does not yet carry a halogensubstituent, this can be introduced in the dioxide by first introducing a nitrogroup, be reducing this, by diazotizing, and subsequently by replacing the diazoniumsalt group by a halogen group with a Sandmeyer reaction. Introduction of a cyano group takes place in an analogous way. Compounds carrying mixed substituents (for instance nitroand halogen) can be obtained by nitrating the halogen carrying compound.

In order to illustrate the invention the preparation of some photoconductive compositions as well as their application for reproduction elements will be described in the following examples.

EXAMPLE I 0.90 g 0.0046 mole) of poly-N-vinylcarbazole, dissolved in 11 ml of tetrahydrofurane, was added to a solution of 0.82 g 0.0023 mole) of 1,3,7-trinitrodibenzothiofene-S,S-dioxide in 10 ml of 1,4-dioxane. The two solutions were stirred whilst they were heated to about 50C. When the mixing was complete, the solution was allowed to cool down to room temperature. By means of this solution an aluminium plate was provided with a light-sensitive layer which after drying had a thickness of about 3-4 1.4.. The reproduction element thus obtained was charged under a positive corona of 7 kV to about 200 V, then it was imagewise exposed under an enlarging apparatus during 2 seconds with a glow lamp, whereby the quantity of light on the irradiated areas amounted to 40 lux, and finally it was developed, for instance with a powdery toner.

This loose powder image could both be fixed on the plate itself and could also first be transferred to a receiving material, such as paper, and be fixed on this. In both cases an excellent image was obtained.

Instead of transfer of the powder image transfer of the latent image itself was also possible. By developing this latent image formed on the receiving material and by fixing, if necessary, the copy was then obtained.

The sensitivity of the above-formed light-sensitive plate, measured on the Victoreen Electrostatic Paper Analyzer via the static method (as described in Tappi 50 (2) 39A and following (1967) with a glow lamp which emits 10 lux on the exposed areas, was such that only 41 lux-sec. were needed in order to reduce the ini tial potential of V on the layer to 25 of this.

EXAMPLE II III To a solution of 0.164 g of l,3,7-trinitro-dibenzothiofene-5,5-dioxide in 10 ml of 1,4-dioxane, while stirring and heating, a solution of 0.90 g of poly-N- vinylcarbazole in 11 ml of tetrahydrofurane was added. The molar proportion donor: acceptor was 10 l in this case. With the aid of this solution an aluminium foil was provided with a light sensitive layer, which after drying had a thickness of 3 5 p. The reproduction element thus obtained was charged under a negative corona to a potential of about 270 V. The energy, necessary to reduce the potential to 25 of the initial potential, was 75 lux sec. For the same layer, when positively charged this value was 100 lux-sec.

When the molar proportion donor acceptor was increased to 20 1, the light-sensitivity slightly decreased, and was about 100 lux sec. for a layer charged negatively and about 105 lux sec. for a layer charged positively.

EXAMPLE IV With the aid of a solution of poly-N-vinylcarbazole and 3,7-dibromo-1,9-dinitro-dibenzothiofene-S,5- dioxide, in which the proportion donor acceptor was 20 1, an aluminium support was coated with a lightsensitive layer having a thickness of 2-3 p.. The number of lux sec., required to reduce the initial potential of 500 V to 125 V 25 upon exposure with a glowlamp amounted to 215 lux sec. for a plate charged negatively and 180 lux sec. for a plate charged positively.

EXAMPLE V An aluminium support was provided with a lightsensitive layer having a thickness of 3 ,u, which contained poly-N-vinylcarbazole as donor and 3,7-dinitrodibenzothiofene-5oxide as acceptor in the molar proportion 1. The reproduction-element thus obtained was charged by means of a negative corona to about 400 V. The 70 light-sensitivity of this layer was 350 lux-sec.

We claim:

1. Electrophotographic reproduction element comprising a support having thereon a photoconductive coating layer containing an organic photoconductive compound that is an electron donor and an organic electron acceptor that forms a donor-acceptor complex with said donor, said acceptor being present in a ratio of between 0.001 and 1 mol thereof per mol of said donor calculated as a monomer, and being a compound of the formula in which n l or 2 and R and R, which may be the same or differentQeach represents one or more hydrogen atoms or one or more substituents selected from the group consisting of halogen atoms and nitro and cyano groups and at least one of R and R represents one or more of said substituents.

2. Electrophotographic reproduction element according to claim 1, in which at least one of R and R represents one or more nitro groups.

3. Electrophotographic reproduction element according to claim 1 in which n 2.

4. Electrophotographic reproduction element according to claim 1, said acceptor being 1, 3, 7-trinitrodibenzothiofene-5,5-dioxide.

5. Electrophotographic reproduction element according to claim 1, said photoconductive compound being selected from the group consistingvof a poly-N- vinyl carbazole, monomeric carbazole compounds, 2,5- bis (4-diaminophenyl)-l,3,4-oxadiazole, 2,5-bis (4- dialkylaminophenyl)-l,3,4-oxadiazole and poly-9-vinyl anthracene.

6. Electrophotographic reproduction element according to claim 1, said photoconductive compound being poly-N-vinyl carbazole and said acceptor being 1,3,7-trinitrodibenZothiofene-S,5-dioxide.

7. Electrophotographic reproduction element according to claim 6, containing from about 1 to about 10 mols of said carbazole per mol of a said acceptor, whereby said element is more light-sensitive when negatively charged than when positively charged.

8. Electrophotographic reproduction element according to claim 6, containing from about 10 to about 25 mols of said carbazole per mol of said acceptor, whereby said element is approximately equally light sensitive when either negatively or positively charged.

9. Electrophotographic reproduction element according to claim 6, containing more than about 25 mols of said carbazole per mol of said acceptor, whereby said element is more light-sensitive when positively charged than when negatively charged.

10. Electrophotographic reproduction element according to claim 1, said layer having a thickness of about 3 to 10 microns, said photoconductive compound being poly-N-vinyl carbazole, said acceptor being 1,3,7-trinitro-dibenzothiofene-S,5-dioxide, and said layer containing from about 1 to about 10 mols of said carbazole per mol of said acceptor.

11. Photoconductive composition for use in a coating layer of an electrophotographic reproduction element, comprising in admixture an organic photoconductive compound that is an electron donor and, per mol of said donor calculated as a monomer, between 0.001 and 1 mol of an organicelectron acceptor that forms a donor-acceptor complex with said donor, said acceptor being a compound of the formula in which n l or 2 and R and R, which may be the same or different, each represents one or more hydrogen atoms or one or more substituents selected from the group consisting of halogen atoms and nitro and cyano groups and at least one of R and R represents one or more of said substituents.

l2. Photoconductive composition accordingto claim 11, said photoconductive compound being selected from the group consisting of poly-N-vinyl carbazole, monomeric carbazole compounds, 2,5-bis(4 diaminophenyl)-1,3,4-oxadiazole, 2,5-bis(4-dialkylaminophenyl)-1,3,4-oxadiazole and poly-9-vinyl anthracene.

13. Photoconductive composition according to claim 11, said acceptor being 1,3,7-trinitrodibenzothiofene- 5,5-dioxide.

14. Photoconductive composition according to claim 11, said photoconductive compound being poly-N- vinyl carbazole and said acceptor being 1,3,7-

trinitrodibenzothiofene-S,S-dioxide. trinitrodibenzothiofene-5,5 dioxide, and about 1 to 15. Photoconductive composition according to claim mols of said photoconductive compound being present per mol of said acceptor.

11, said photoconductive compound being poly-N- viny] carbazole, said acceptor being 1,3,7-

Patent No.

UNITED STATES PATENT OFFICE Dated September 16, 1975 Line Line

line

Line Line line [SEAL] Inventor(s) Gerardus J. Crommentuyn et al.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

"produces" should read -products-.

"0:58" should read O.58.

"which" should read with--.

"0:98" should read -0.98.

"diobromo" should read dibromo-. "sutstituents" should read substituents.

"0:02" should read --0.02.

after "I" delete "on".

"dinitrocompounds" should read --dinitrocompounds- "he" should read by-.

"whilst" should read while-.

"270V." should read -270 V..

Signed and Scaled this sixth Day of April1976 Attesl.

RUTH C. MASON A nesting Officer 

1. ELECTROPHOTOGRAPHIC REPRODUCTION ELEMENT COMPRISING A SUPPORT HAVING THEREON A PHOTOCONDUCTIVE COATING LAYER CONTAINING AN ORGANIC PHOTOCONDUCTIVE COMPOUND THAT IS AN ELECTRON DONOR AND AN ORGANIC ELECTRON ACCEPTOR THAT FORMS A DONOR-ACCEPTOR COMPLEX WITH SAID DONOR, SAID ACCEPTOR BEING PRESENT IN A RATIO OF BETWEEN 0.001 AND 1 MOL THEREOF PER MOL OF SAID DONOR CALCULATED AS A MONOMER, AND BEING A CMPOUND OF THE FORMULA
 2. Electrophotographic reproduction element according to claim 1, in which at least one of R and R1 represents one or more nitro groups.
 3. Electrophotographic reproduction element according to claim 1 in which n
 2. 4. Electrophotographic reproduction element according to claim 1, said acceptor being 1, 3, 7-trinitro-dibenzothiofene-5,5-dioxide.
 5. Electrophotographic reproduction element according to claim 1, said photoconductive compound being selected from the group consisting of a poly-N-vinyl carbazole, monomeric carbazole compounds, 2,5-bis (4-diaminophenyl)-1,3,4-oxadiazole, 2,5-bis (4-dialkylaminophenyl)-1,3,4-oxadiazole and poly-9-vinyl anthracene.
 6. Electrophotographic reproduction element according to claim 1, said photoconductive compound being poly-N-vinyl carbazole and said acceptor being 1,3,7-trinitrodibenzothiofene-5,5-dioxide.
 7. Electrophotographic reproduction element according to claim 6, containing from about 1 to about 10 mols of said carbazole per mol of a said acceptor, whereby said element is more light-sensitive when negatively charged than when positively charged.
 8. Electrophotographic reproduction element according to claim 6, containing from about 10 to about 25 mols of said carbazole per mol of said acceptor, whereby said element is approximately equally light-sensitive when either negatively or positively charged.
 9. Electrophotographic reproduction element according to claim 6, containing more than about 25 mols of said carbazole per mol of said acceptor, whereby said element is more light-sensitive when positively charged than when negatively charged.
 10. Electrophotographic reproduction element according to claim 1, said layer having a thickness of about 3 to 10 microns, said photoconductive compound being poly-N-vinyl carbazole, said acceptor being 1,3,7-trinitro-dibenzothiofene-5,5-dioxide, and said layer containing from about 1 to about 10 mols of said carbazole per mol of said acceptor.
 11. Photoconductive composition for use in a coating layer of an electrophotographic reproduction element, comprising in admixture an organic photoconductive compound that is an electron donor and, per mol of said donor calculated as a monomer, between 0.001 and 1 mol of an organicelectron acceptor that forms a donor-acceptor complex with said donor, said acceptor being a compound of the formula
 12. Photoconductive composition according to claim 11, said photoconductive compound being selected from the group consisting of poly-N-vinyl carbazole, monomeric carbazole compounds, 2,5-bis(4-diaminophenyl)-1,3,4-oxadiazole, 2,5-bis(4-dialkylaminophenyl)-1,3,4-oxadiazole and poly-9-vinyl anthracene.
 13. Photoconductive composition according to claim 11, said acceptor being 1,3,7-trinitrodibenzothiofene-5,5-dioxide.
 14. Photoconductive composition according to claim 11, said photoconductive compound being poly-N-vinyl carbazole and said acceptor being 1,3,7-trinitrodibenzothiofene-5,5-dioxide.
 15. Photoconductive composition according to claim 11, said photoconductive compound being poly-N-vinyl carbazole, said acceptor being 1,3,7-trinitrodibenzothiofene-5,5 dioxide, and about 1 to 10 mols of said photoconductive compound being present per mol of said acceptor. 